1. Field of the Invention
The present invention relates generally to diffractive light modulators and, more particularly, to an interdigitation-type diffractive light modulator in which each of a pair of ribbons has a plurality of diffractive branches arranged in a comb shape, and in which the diffractive branches of the ribbons interdigitate with each other, and the respective ribbons move upwards and downwards or, alternatively, one ribbon moves upwards and downwards, so that the diffractive branches of the ribbons which interdigitate with each other form a stepped structure, thus diffracting incident light.
2. Description of the Related Art
Recently, with the development of micro-technology, MEMS (micro-electro-mechanical systems) devices and micro-apparatuses to assemble the MEMS devices are gaining popularity.
The MEMS devices form a microstructure on a substrate, such as a silicon substrate and a glass substrate, in which an actuator to generate a mechanical actuating force and a semiconductor integrated circuit to control the actuator are electrically and mechanically assembled together. The fundamental feature of the MEMS device is that the actuator having a mechanical structure is provided at a predetermined position of the MEMS device and is electrically operated by using Coulomb force generated between electrodes.
FIGS. 1 and 2 show representative optical MEMS devices for light activated switches and light modulator elements using the reflection or diffraction of light.
An optical MEMS device 1 shown in FIG. 1 includes a substrate 2, a substrate-side electrode 3 which is placed on the substrate 2, and a cross-beam 6 which has an actuator-side electrode 4 that is arranged parallel to the substrate-side electrode 3. The actuator-side electrode 4 of the cross-beam 6 corresponds to the substrate-side electrode 3 of the substrate 2. The optical MEMS device further includes a support 7 which is coupled to an end of the cross-beam 6 to support the cross-beam 6. The cross-beam 6 is electrically insulated from the substrate-side electrode 3 by a gap 8 defined between them.
In this optical MEMS device 1, the cross-beam 6 is moved by electrostatic attraction which is generated by electric potential applied to the substrate-side electrode 3 and the actuator-side electrode 4. For example, as shown by the real line and the broken line in FIG. 1, the cross-beam 6 moves from a state of being parallel to the substrate-side electrode 3 to a state of being inclined with respect to the substrate-side electrode 3.
Another optical MEMS device 11 is shown in FIG. 2. This optical MEMS device 11 includes a substrate 12, a substrate-side electrode 13 which is placed on the substrate 12, and a bridge-shaped beam 14 which is laid across the substrate-side electrode 13. The bridge-shaped beam 14 is electrically insulated from the substrate-side electrode 13 by a gap 10 defined between them.
The bridge-shaped beam 14 has a bridge body 15 and an actuator-side electrode 16 which is provided on the bridge body 15 parallel to the substrate-side electrode 13 to correspond to the substrate-side electrode 13.
In this optical MEMS device 11, according to electric potential applied to the substrate-side electrode 13 and the actuator-side electrode 16, the bridge-shaped beam 14 is moved by electrostatic attraction generated between the substrate-side electrode 13 and the actuator-side electrode 16. That is, the bridge-shaped beam 14 moves from a state of being parallel to the substrate-side electrode 13 to a state of being bent toward the substrate-side electrode 13.
These optical MEMS devices 1 and 11 may be used as light activated switches which serve to detect light reflected in a predetermined direction using the phenomenon that, when light is radiated onto a surface of the actuator-side electrode 4 or 16 serving as a light reflective surface, the direction of the reflected light varies according to the location of the beam 6 or 14.
Alternatively, the optical MEMS devices 1 and 11 may be used as light modulator elements to modulate light intensity. In the use of reflection of light, the light intensity is modulated by vibrating the beam 6 or 14 and varying the amount of light reflected in a predetermined direction per hour.
To use diffraction of light, a plurality of beams 6 or 14 is arranged parallel to the substrate-side electrode 3 or 13 to form a light modulator element. For example, alternate beams 6 or 14 move toward or away from the substrate-side electrode 3 or 13, so as to change the height of the actuator-side electrodes serving even as the light reflective surface, thus diffracting incident light. The intensity of light, reflected by the actuator-side electrodes, is modulated through the above-mentioned light diffraction. The light modulation by this light modulator element is a spatial modulation.
FIG. 3a and FIG. 3b shows the construction of a GLV (grating light valve) device which is a light intensity control unit, that is, a light modulator, and which was developed by SLM (silicon light machine) corporation.
As shown in FIG. 3a and FIG. 3b, the GLV device 21 includes a substrate-side electrode 23 which is placed on an insulated substrate 22 such as a glass substrate, and a plurality of beams 24 (241, 242, 243, 244, 245 and 246) which are laid across the substrate-side electrode 23 parallel to each other in a bridge shape.
Each beam 24 including both a bridge body 25 and an actuator-side electrode 26, which is provided on the bridge body 25 and serves as a reflective surface, is called a ribbon.
When a small amount of voltage is applied between the substrate-side electrode 23 and the actuator-side electrodes 26 serving as the reflective surface, alternate beams 24 are moved towards the substrate-side electrode 23 by electrostatic force. When the application of voltage is stopped, the beams 24 are moved away from the substrate-side electrode 23, thus being returned to the initial state.
In the GLV device 21, the height of alternate actuator-side electrodes 26 is varied by the movement of the alternate beams 24 towards or away from the substrate-side electrode 23. The diffraction of light modulates the intensity of light reflected by the actuator-side electrodes 26 (here, one light support is radiated to all of the six beams 24).
In conventional GLV devices, because, in the case in which only two ribbons form one pixel, diffraction efficiency is low, four or six ribbons form one pixel. Furthermore, to increase the diffraction efficiency of a GLV device, more ribbons are required. However, this increases manufacturing costs and requires a large area. Thus, conventional GLV devices have reached the limit of miniaturization.
Furthermore, in conventional GLV devices, because light is diffracted towards ribbons, when the ribbons are arranged in a linear array, it is difficult to distinguish between a zero order diffraction and a first order diffraction.